Can you clarify what you mean about them attempting to apply their moderation policies to another website then? The rest of the context makes this appear as if you mean the NY Post...
The content in question was not hosted on Twitter, it was hosted on the NY Post website. Twitter moderates the content hosted on Twitter, the NY Post moderates the content hosted by the NY Post. But when Twitter censored links to the NY Post, Twitter was attempting to apply its own moderation policy to content hosted outside of Twitter.
Again, I'm not claiming that blocking links is inherently bad. For example, it may be reasonable to block links to malware, phishing, misleading URL shorteners, etc. I'm just saying that this is censorship. "Moderation" is a euphemism for censorship.